1. Field of the Invention
The present invention relates to semiconductor lasers and more particularly to temperature compensation techniques for slope efficiency.
2. Description of the Prior Art
For most of the history of semiconductor lasers, both the current threshold Ith and the slope efficiency xcex7 decreased monotonically and generally exponentially with increasing temperature of the active material. With the advent of VCSELs, it has been possible to vary the tuning of the cavity resonance with respect to the peak of the optical gain in order to produce a non-monotonic variation in the Ith which is most simply approximated by a quadratic function. This technique generally referred to as xe2x80x9cgain offsetxe2x80x9d is well known in the VCSEL field. The VCSEL design parameters are generally set such that the variation in threshold is minimized over the operating temperature range. Gain offset has been demonstrated to a lesser degree in edge-emitting DFB lasers. Gain offset is not applicable to Fabry-Perot semiconductor lasers whose effective cavity length is about 10 xcexcm or more. In all these lasers, and in fact in all known semiconductor lasers, the slope efficiency still decreases monotonically with temperature and is only negligibly, e.g. only to 2nd order, affected by gain offset. The temperature variation of the slope efficiency dxcex7/dT has been governed by quantum mechanical confinement of the electrons and holes in the active region.
In prior art devices, there has been no effective method for controlling temperature variation of the slope efficiency dxcex7/dT. The only method utilized by prior art devices is to optimize the quantum mechanical confinement within the limitations of the material system. This approach is generally already used to its maximum practical limit. It only reduces the decay of slope efficiency with temperature, but its effect is limited by the availability of material structures that are consistent with the desired laser emission wavelength. Furthermore, optimizing for quantum mechanical confinement may involve compromises with other aspects of the laser such as electrical resistance or manufacturability.
It is therefore an object of the present invention to provide a method and device for controlling temperature variation of the slope efficiency dxcex7/dT.
It is a further object to provide a semiconductor laser having at least one mirror whose reflectivity or transmissivity, at the laser wavelength, varies with temperature.
It is yet another object to provide a semiconductor laser having an output power which is stable over an operating region when the temperature of the laser fluctuates.
It is yet another object to provide a semiconductor laser having a flattened slope efficiency vs. temperature which is controlled by optical means.
In all of the above embodiments, it is an object to provide a robust semiconductor laser having temperature stability that is provided by optical means.
Finally, it is an object of the invention to provide a semiconductor laser having at least one mirror structure whose change in reflectivity or transmissivity, is used to compensate for temperature change in the semiconductor laser.
According to one broad aspect of the present invention, there is provided a semiconductor laser, the semiconductor laser having an emission wavelength and comprising: a light emitting material; means for causing the light emitting material to emit light; at least first and second mirrors disposed on opposite sides of the light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through the second mirror; and optical means for compensating the semiconductor laser for temperature variations in the slope of output power vs. input power of the semiconductor laser.
According to another broad aspect of the invention, there is provided a semiconductor laser, the semiconductor laser having a first emission wavelength and comprising: a light emitting material; means for causing the light emitting material to emit light; and at least first and second mirrors disposed on opposite sides of the light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through the second mirror, the second mirror having a first reflectivity at a first emission wavelength at a first temperature, and a second reflectivity lower than the first reflectivity at a second emission wavelength at a second temperature higher than the first temperature.
According to yet another broad aspect of the invention, there is provided a semiconductor laser, the semiconductor laser having a first emission wavelength and comprising: a light emitting material; means for causing the light emitting material to emit light; and at least first and second mirrors disposed on opposite sides of the light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through the second mirror, the first mirror having a first reflectivity at a first emission wavelength at a first temperature, and a second reflectivity lower than the first reflectivity at a second emission wavelength at a second temperature higher than the first temperature.
According to yet another broad aspect of the invention, there is provided a semiconductor laser, the semiconductor laser having a first emission wavelength at a first temperature and comprising: a light emitting material; means for causing the light emitting material to emit light; and at least first and second mirrors disposed on opposite sides of the light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through the second mirror, wherein either or both of the first or second mirrors has at least a portion which has a respective local minimum in reflectivity disposed between two respective maximums in reflectivity the respective local minimum disposed near the emission wavelength at a first temperature.
According to yet another broad aspect of the invention, there is provided a semiconductor laser, the semiconductor laser having a first emission wavelength at a first temperature and comprising: a light emitting material; means for causing the light emitting material to emit light; at least first and second mirrors disposed on opposite sides of the light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through the second mirror, wherein the first mirror has a first transmissivity at the first emission wavelength at the first temperature, the second mirror has a second transmissivity at the first emission wavelength at the first temperature, the first mirror has a third transmissivity at a second emission wavelength at a second temperature higher than the first temperature, the second mirror has a fourth transmissivity at the second emission wavelength at the second temperature, a ratio of the fourth transmissivity to the third transmissivity being larger that a ratio of the second transmissivity to the first transmissivity; and optical means for compensating the semiconductor laser for temperature variations in the slope of the output power vs. input power of the semiconductor laser.
According to yet another broad aspect of the invention, there is provided a semiconductor laser, said semiconductor laser having a first emission wavelength at a first temperature and comprising: a light emitting material; means for causing said light emitting material to emit light; at least first and second mirrors disposed on opposite sides of said light emitting material thereby forming an optical cavity inside which lasing occurs and from which output power is emitted through said second mirror, wherein said first mirror comprises at least first and second respective layers, said first layer comprising a high-index material and said second layer comprising a low-index material, said first layer having a xcex94n/xcex94T which is greater than a xcex94n/xcex94T associated with said second layer; and optical means for compensating said semiconductor laser for temperature variations in the slope of the output power vs. input power of said semiconductor laser.
Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiment.